Add "Background FGS start" capability and change SHORT FGS procstate

- SHORT_SERVICE FGS now uses the FGS procstates, just like other FGS types.
  This ensures SHORT_FGS will get all "special powers" of FGS -- e.g.
  ability to hold on to wake locks during doze, ability to defeat
  Data Saver.

- However, we still shouldn't allow BFSL ("background foreground service
  lauch"), so add a new "BFSL" capability.

- Unfortunately, some of the CTS tests would break if we just add
  new capability, so the new capability is "hidden" from `am watch-uids`
  by default.

- PROCESS_CAPABILITY_ALL is also removed from @TestApi, to avoid exposing
  the new capability to CTS.

Test: atest FrameworksServicesTests:OomAdjusterTests MockingOomAdjusterTests
Test: atest CtsShortFgsTestCases \
    ActivityManagerFgsBgStartTest \
    ActivityManagerApi29Test \
    tests/app/src/android/app/cts/ActivityManagerProcessStateTest.java \
    /android/master2/cts/tests/app/src/android/app/cts/ActivityManagerTest.java \
    ExpeditedJobTest

(ExpeditedJobTest#testTopEJUnlimited was already failing)

Bug: 257270313
Change-Id: I838da70030811ed719cba9c147c5b42d2e8601f3